Physiological and nutritional stress response of African elephants within the lantana-dominated Lower Imenti Forest Reserve in Kenya

Invasive species can alter the ecology of protected areas, substantially lowering the habitat quality for vertebrate communities. The Lower Imenti Forest on Mt. Kenya’s northeastern slope has experienced habitat disturbance, degrading the system and resulting in the establishment of invasive species, including lantana (Lantana camara), throughout the area. Following reports of high mortality and poor conditions among the African savanna elephants (Loxodonta africana) inhabiting the area, we assessed the status of two endocrine indicators of their physiological condition. Specifically, we assessed the physiological stress response by measuring faecal glucocorticoid metabolites (fGCM) and the nutritional stress response by measuring faecal thyroid (fT3) concentrations in elephant faecal samples collected in the forest. To better interpret the hormone levels, we compared the hormone concentrations in the Imenti faecal samples to concentrations from reference levels indicative of extreme nutritional stress (from faecal samples of elephants experiencing drought-induced mortality) and adrenal stress (from elephants experiencing high levels of human–elephant conflict). The concentrations of fT3, a biomarker of nutritional stress response, found in elephant faecal samples from the Lower Imenti Forest were lower than the drought-stressed reference levels, suggesting lower levels of energy intake and assimilation of forage resources in elephants from this area. The concentration of fGCM, a biomarker of physiological stress response, was higher than the human–elephant conflict reference levels, suggesting the elephants in Lower Imenti were experiencing a higher physiological stress response. We found no differences between fT3 and fGCM concentrations in samples assigned to different age classes (juvenile, subadults, adults), suggesting the physiological problems were not age specific. Findings from our physiological study suggest that restricted movement and reduced forage availability due to lantana infestation in the Lower Imenti Forest may be driving the elevated nutritional stress, potentially contributing to the concerning mortality observed in the area. We discuss the use of endocrine markers to ascertain wildlife responses to degraded habitats.